1. Field of the Invention
The present invention pertains to seismic surveying and, more particularly, to predicting currents during the deployment of survey equipment.
2. Description of the Related Art
Seismic exploration is conducted on both land and in water. In both environments, exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying acoustic source(s) and acoustic sensors at predetermined locations. The sources impart acoustic waves into the geological formations. Features of the geological formation reflect the acoustic waves to the sensors. The sensors receive the reflected waves, which are detected, conditioned, and processed to generate seismic data. Analysis of the seismic data can then indicate probable locations of the hydrocarbon deposits.
Accurate knowledge of source and sensor positions is important to the accuracy of the analysis. In land surveys, accurate positioning is not particularly difficult because environmental conditions are usually relatively stable. Sources and sensors can be readily positioned where desired and, once placed, they usually do not shift to any great degree. Marine surveys, however, are different altogether. Marine surveys come in at least two types. In a first, an array of streamers and sources is towed behind a survey vessel. In a second type, an array of seismic cables, each of which includes multiple sensors, is laid on the ocean floor, or sea bottom, and a source is towed from a survey vessel. In both cases, many factors complicate determining the position of the sensors, including wind, currents, water depth, and inaccessibility.
One increasingly common marine seismic survey technique is known as “time-lapse seismic surveying.” This technique essentially repeats earlier surveys over time to reveal changes in reservoirs of hydrocarbon deposits. One way to do this is to position the acoustic source(s) and receivers as close as is reasonably practicable to the positions of corresponding acoustic source(s) and receivers in the earlier survey(s).
Among the complicating factors mentioned above, ocean currents figure prominently. Currents may vary significantly in both direction and strength over the course of a marine seismic survey. Consider, for instance, a typical towed-array survey in which a vessel tows 8 streamers, each 6 km long and separated by 100 m. At any given instant, the survey covers 4.2 km2. The survey vessel then will typically tow the streamers back and forth over distances of, for example, 120 km. Thus, the survey will cover quite a large area, and the currents within the survey area may vary dramatically. Or, in an ocean bottom survey, much attention is paid to the positioning of the seismic cables as they are laid. Control over the positioning helps optimize the deployment speed and accuracy and avoids tangling the seismic cable with other obstructions, such as other cables or sub-sea devices. However, currents may very greatly at different depths. The seismic cables are subjected to complexly varying currents as they descend through the water column to the seabed.
Thus, the ability to predict or project what the currents will do in the near future is greatly valued. If the surveyor knows what the currents will do, they can proactively act to offset undesirable effects of the currents. For instance, in a towed-array survey, the surveyor can steer deflectors, birds, or other steerable elements of the array to maintain the desired position for the streamers. Similarly, in an ocean bottom survey, the surveyor could steer the vessel as the cables are deployed to help offset drift induced by currents. Thus, one can use knowledge of incoming currents to mitigate positioning errors before they occur—a form of feed forward control—rather than waiting for the errors to occur and then correcting them—a form of feedback control.
Current techniques apply various modeling techniques to project the shape and/or position of the seismic cable during deployment. These models consider the physical characteristics of the seismic cable (e.g., weight, diameter, etc.) and account for the effect of predicted sea currents on the seismic cable as it descends to the sea floor. However, such methods provide only a model, or projection, of the seismic cable's shape and are predicated on a limited knowledge of the sea's properties.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.